SAO PAULO — Ronaldo retired from soccer Monday because his body can no longer meet the demands of the game, ending a magnificent career in which he won two World Cups with Brazil and thrived with some of Europe's top teams.

Ronaldo retires soccer star Ronaldo looks on during a press conference in Sao Paulo, Brazil, Monday, Feb. 14, 2011. Ronaldo, 34,said he is retiring from soccer because he can't stay fit anymore, ending a stellar 18-year career in which he thrived with Brazil and some of Europe's top clubs.With tears on his cheeks, the 34-year-old striker announced his decision at a news conference. He played 18 years despite repeated knee trouble and leaves as a three-timer FIFA Player of the Year and the top goal scorer in World Cup history.

"It's very hard to leave something that made me so happy," said Ronaldo, with sons Alex and Ronald by his side. "Mentally, I wanted to continue, but I have to acknowledge that I lost (the fight) to my body."

Now he confronts a new stage, shadowed by all that's left behind.

"With this announcement," he said, "it feels like it's my first death."

Ronaldo said a string of injuries the past two years kept him from performing at a high level with the Brazilian club Corinthians. He also learned four years ago he had hypothyroidism, a condition that made it difficult for him to lose weight and stay in shape.

"The pain made me anticipate the end of my career," he said.

With his uncanny sprints toward the goal and an extraordinary finishing touch, Ronaldo captivated fans everywhere he played, including Barcelona, Real Madrid and Inter Milan. All despite having to come back from three serious knee injuries that threatened to end his career.

"My career was beautiful, was wonderful," he said. "I've had many defeats but infinite victories."

Ronaldo's contract with Corinthians was expected to end at the end of the year, but his condition kept deteriorating. He was visibly out of shape and fans jeered him after every missed play.

The announcement came days after he was heavily criticized by Corinthians fans for the team's elimination from the Copa Libertadores, Latin America's most important competition and the only major tournament the popular club has yet to win.

Fans damaged players' cars and threw rocks at the team bus, but Ronaldo said that did not play a part in his retirement. With his voice cracking, he said he wanted to only thank Corinthians fans for their support.

"I've never seen fans with so much passion," he said. "Their need for results sometimes made them a little aggressive, a bit out of control."

Ronaldo's exit comes two days after former Brazil teammate Roberto Carlos left Corinthians because he and his family were being threatened by fans after the Copa Libertadores elimination. Corinthians players had already been under pressure since last year, when the club failed to win titles despite celebrating its centenary.

"I want to publicly apologize for failing in the Libertadores project," he said, crying, before team President Andres Sanchez handed him a jersey with the words "forever" and "phenomenon."

The last of Ronaldo's more than 400 career goals came with a penalty kick in a 1-0 win over Cruzeiro in the Brazilian league on Nov. 13. The last match was the 2-0 Copa Libertadores loss to Deportes Tolima on Feb. 2 in Colombia.

Ronaldo was part of the World Cup squad that won the 1994 title in the United States, although he was a teenager and never played. He was the team's top star in France in 1998, but just before the final he had seizures in the team's hotel and did not play well in the game won by the French.

"Ronaldo is the best player I ever played with," former Inter Milan teammate Youri Djorkaeff, who also played against Ronaldo in the 1998 World Cup final, told French RMC radio. "But he was also a very engaging person. When we were training, he would always come up with crazy dribbles. We would practically stop to watch him. It was extraordinary."

It was in 2002 that Ronaldo peaked with the national team, helping Brazil win the tournament in South Korea and Japan with two goals in the final against Germany. He was voted Player of the Year, adding to the awards won in 1996 and 1997.

He was among those who disappointed in the 2006 World Cup in Germany, when Brazil fell to the French again in the quarterfinals, but he was still able to score his record-breaking 15th goal, a mark that still stands.

Ronaldo also helped Brazil win the 1997 and 1999 Copa America, as well as the 1997 Confederations Cup and a bronze medal at the 1996 Atlanta Olympics.

Ronaldo scored 67 goals in 104 games with Brazil, but was never again called up to the national team after the disappointing performance in 2006. There were calls for his return to the squad after he helped Corinthians win the 2009 Brazilian Cup, but he was never able to get back in shape.

Ronaldo's career was marked by his goals just as it was by his injuries, which kept him sidelined for nearly three years in total.

He tore up his right knee with Inter Milan in 1999 and needed surgery, and a year later he twisted the same knee on the day he was returning to action and had to be sidelined for several months again after another surgery. The third injury came with AC Milan in 2008, forcing another surgery and another long layoff.

Ronaldo began his professional career with Cruzeiro in 1993 as a 16-year-old, and it didn't take long for him to start stunning fans across Brazil. He moved to PSV Eindhoven that year, becoming the club's top scorer and earning a transfer to Barcelona in 1996.

He quickly became an idol at the Spanish club, scoring 34 goals in 37 matches in the Spanish league and helping the club win the Copa del Rey. A year later he signed with Inter Milan, winning the 1998 UEFA Cup and earning the nickname "The Phenomenon."

He moved to Real Madrid in 2002, one of the few players revered by fans of both Spanish powerhouses. He helped Real Madrid win the Spanish league in 2003 and 2007. After that season he joined AC Milan, but his third knee injury cut short his stay.

Ronaldo is a goodwill ambassador for the United Nation's Development Program. He played a big role in organizing the "Game for Peace" in Haiti in 2004, when the Brazilians visited the country devastated by gang wars.